I. Field of the Invention
The present invention relates to fluid devices and particularly to fluid devices of the variable displacement, axial piston type which may function either as a fluid motor or as a fluid pump.
II. Description of the Prior Art
Heretofore, fluid pumping and fluid motoring devices of the axial piston type have been constructed of a suitable metal housing having a revolving cylinder barrel provided with a plurality of parallel cylinder bores therein and within which pistons are reciprocally mounted. The pistons are reciprocated by engagement with a thrust plate assembly or the like. A rotary valve mechanism in the form of cylinder ports disposed at end of the cylinder barrel alternately connects each cylinder bore with inlet and outlet passages of the device as the cylinder barrel is rotated.
The thrust plate assembly in fluid devices of the variable displacement type normally takes the form of a yoke having transversely extending pintles rotatably carried in bearings suitably mounted in the wall of the housing. Suitable means are provided to pivot the thrust plate with respect to the longitudinal axis of the drive shaft on which the cylinder barrel is rotated so as to vary the amount of reciprocal movement imparted to the pistons within the cylinder bores and thereby permit a selected variation in the fluid displaced by such axial piston fluid devices. Since the bearings supporting the yoke are mounted to the wall of the housing, the entire force exerted against the thrust plate assembly due to the fluid pressure acting against the pistons within the cylinder barrel bores is taken by the housing. This necessitates a strong metal housing requiring considerable precision of manufacture and results in a larger unit than may be necessary for the displacement that is desired. One attempt to overcome the difficulties encountered in such constructions is disclosed in my U.S. Pat. No. 3,991,658 and No. 3,868,889.
As speed and pressure are increased in fluid devices of the type described, there has always been an accompanying increase in noise. This general increase in noise with increased speed and pressure may be attributed to a number of factors and devices of the axial piston type. First, the frequencies generated by the device increase with speed as the components of the device are subjected to increased, alternating, impact forces; second, the intensity of speed related sounds increases as the impact forces between the components of the device increase; and third, the excitation spectrum of the significant piston harmonics also broadens, thus increasing the number of resonant responses. It would be desirable to provide a fluid piston device wherein the attendant noises and vibration levels may be significantly reduced. The present invention achieves this by providing a novel arrangement of components and a technique for assembling the same so as to achieve a compact unit.
In axial piston devices of the type described, when the same are operating under pressure, certain related areas are so proportioned that the cylinder barrel is positively biased toward the valve plate by that pressure in a manner which is well known to those skilled in the art of such units. For purposes of starting the unit, however, it is necessary that the valve be mechanically biased against the valve plate, and this is normally accomplished by means of a spring disposed between the cylinder barrel and the thrust plate. In such devices the normal thrust component or side thrust of the piston not only creates the driving torque, but also results in a substantial lateral force on the cylinder barrel tending to displace it from its normal position. In counter acting this lateral force, it is important that the cylinder barrel be maintained flatly against the valve plate or the cylinder barrel will lift off completely due to fluid flow conditions between the valve plate and the cylinder barrel interface; and this, of course, renders the device inoperative or may result in scoring of the valve plate and/or cylinder barrel faces. In prior art devices of this type support for the cylinder barrel against lateral displacement has been provided by two general schemes. The first of these is to provide a radial bearing for the cylinder barrel directly interposed between the cylinder barrel and the housing. Such construction is disclosed in the aforementioned United States patents. The second scheme of construction has been to support the cylinder barrel directly on the shaft at the driving connection therewith and transmit the lateral thrust thereon to the housing through the shaft and the shaft support. The second of these schemes has advantages in that the size and weight of the units can be reduced. However, in the past when the cylinder barrel was shaft supported, the means for biasing the cylinder barrel into engagement with the valve plate and for biasing the pistons outwardly against the thrust plate were generally unsatisfactory. In such constructions the cylinder barrel, which runs in an abutting fluid sealing relationship with the stationary valve plate, is constantly pressed toward engagement with the valve plate, and the fluids flowing across the face of the valve plate between the cylinder barrel and the valve plate face provide a hydrostatic fluid film which supports the cylinder barrel. In order to prevent excessive wear or galling of the valve plate face and the cylinder barrel face, applicant provides a unique relationship between the cylinder barrel and drive shaft splines that permits a limited and controlled amount of movement of the cylinder barrel, which is both angular and axial, with respect to the drive shaft so that the cylinder barrel may find its own natural center and freely float on the hydrostatic balanced fluid film and hydrokinetic lubricating film formed between the faces of the cylinder barrel and valve plate. Relevant art with respect to applicant's invention is disclosed in my U.S. Pat. No. 3,890,882.
In designing axial piston pumps of the type described herein, it is necessary for the efficient operation of the axial piston pump that the cylinder barrel always be in contact with the valve plate. As aforementioned, any tendency or any lifting or separation of the space between the cylinder barrel and the valve plate causes considerable flow of fluid between the interface of the cylinder barrel and the valve plate. This results in a considerable reduction of volumetric efficiency, as well as the possibility of the scoring of the valve plate cylinder block interface. If the aforementioned cylinder barrel side load in this type of configuration is transmitted to the shaft, the shaft will, of course, naturally deflect. Conventional pump designs require that the deflection of the shaft be first minimized and, second, that the point of maximum shaft deflection should occur at the midpoint along the axis of the spline. In the present invention the angle at which the spline axis assumes relative the cylinder barrel becomes unimportant. This is due to the fact that the cylinder barrel side load is taken through an enlarged diameter portion on the barrel shaft, and this relationship results in the cylinder barrel being centered with respect to the shaft at all times. Additionally, a considerable amount of deflection can be had by providing an increased clearance between the male and female splines of the cylinder barrel and drive shaft. Examples of prior art apparatuses having structural features similar to the aforementioned are disclosed in U.S. Pat. No. 3,126,835 and No. 3,160,109.
As aforementioned, current pump design technology teaches that the point of contact at which the side load of the cylinder barrel is transmitted to the drive shaft should correspond with the intersection of a plane passing through the piston balls and the drive shaft axis. In the present inventive design the plane passing through the piston ball centers intersects the shaft line at a point offset away from the cylinder barrel and valve plate. This creates an imbalance tending to lift the cylinder barrel away from the valve plate. The imbalance is corrected by proper balancing of the cylinder barrel valve plate interface, and the offset results in the substantial improvement, reducing the overall length of the cylinder barrel and resulting in a smaller overall pump length. The smaller pump length makes it possible to move the load supporting bearings, which are attached to the shaft, closer to each other. Bringing these bearings closer together reduces the amount of stress and deflection on the pump shaft, allowing the shaft to be made smaller and, thus, further reducing the required size of the pump for a particular displacement. Additionally, smaller and more compact units result in a quiet pump. In axial piston pumps where the cylinder barrel side load is transmitted to the shaft through the spline, it is necessary to minimize the amount of shaft deflection in order to minimize the tendency on the part of the shaft deflection to lift the cylinder barrel off the valve plate face. To accomplish this, current pump designs must have very bulky shafts and care must be exercized to establish the maximum shaft deflection point at a point that is close to the middle of the spline. By utilizing the aforementioned loose-fitting relationship between the cylinder barrel spline and the drive shaft spline, the shaft deflection becomes unimportant.
U.S. Pat. No. 3,866,520 is relevant to applicant's invention.
III. Prior Art Statement
In the opinion of applicant and applicant's attorney the aforementioned United States patents represent the closest prior art of which applicant and applicant's attorney are aware.